Network Working Group D. Crocker
Request for Comments: 5598 Brandenburg InternetWorking
Category: Informational July 2009
Internet Mail Architecture
Abstract
Over its thirty-five-year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. To collaborate productively on
this large and complex system, all participants need to work from a
common view of it and use a common language to describe its
components and the interactions among them. But the many differences
in perspective currently make it difficult to know exactly what
another participant means. To serve as the necessary common frame of
reference, this document describes the enhanced Internet Mail
architecture, reflecting the current service.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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RFC 5598 Email Architecture July 20091. Introduction
Over its thirty-five-year history, Internet Mail has changed
significantly in scale and complexity, as it has become a global
infrastructure service. These changes have been evolutionary, rather
than revolutionary, reflecting a strong desire to preserve both its
installed base and its usefulness. Today, Internet Mail is
distinguished by many independent operators, many different
components for providing service to Users, as well as many different
components that transfer messages.
The underlying technical standards for Internet Mail comprise a rich
array of functional capabilities. These specifications form the
core:
* Simple Mail Transfer Protocol (SMTP) ([RFC0821], [RFC2821],
[RFC5321]) moves a message through the Internet.
* Internet Mail Format (IMF) ([RFC0733], [RFC0822], [RFC2822],
[RFC5322]) defines a message object.
* Multipurpose Internet Mail Extensions (MIME) [RFC2045] defines
an enhancement to the message object that permits using
multimedia attachments.
Public collaboration on technical, operations, and policy activities
of email, including those that respond to the challenges of email
abuse, has brought a much wider range of participants into the
technical community. To collaborate productively on this large and
complex system, all participants need to work from a common view of
it and use a common language to describe its components and the
interactions among them. But the many differences in perspective
currently make it difficult to know exactly what another participant
means.
It is the need to resolve these differences that motivates this
document, which describes the realities of the current system.
Internet Mail is the subject of ongoing technical, operations, and
policy work, and the discussions often are hindered by different
models of email-service design and different meanings for the same
terms.
To serve as the necessary common frame of reference, this document
describes the enhanced Internet Mail architecture, reflecting the
current service. The document focuses on:
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* Capturing refinements to the email model
* Clarifying functional roles for the architectural components
* Clarifying identity-related issues, across the email service
* Defining terminology for architectural components and their
interactions
1.1. History
The first standardized architecture for networked email specified a
simple split between the user world, in the form of Message User
Agents (MUAs), and the transfer world, in the form of the Message
Handling Service (MHS), which is composed of Message Transfer Agents
(MTAs) [RFC1506]. The MHS accepts a message from one User and
delivers it to one or more other Users, creating a virtual MUA-to-MUA
exchange environment.
As shown in Figure 1, this architecture defines two logical layers of
interoperability. One is directly between Users. The other is among
the components along the transfer path. In addition, there is
interoperability between the layers, first when a message is posted
from the User to the MHS and later when it is delivered from the MHS
to the User.
The operational service has evolved, although core aspects of the
service, such as mailbox addressing and message format style, remain
remarkably constant. The original distinction between the user level
and transfer level remains, but with elaborations in each. The term
"Internet Mail" is used to refer to the entire collection of user and
transfer components and services.
For Internet Mail, the term "end-to-end" usually refers to a single
posting and the set of deliveries that result from a single transit
of the MHS. A common exception is group dialogue that is mediated
through a Mailing List; in this case, two postings occur before
intended Recipients receive an Author's message, as discussed in
Section 2.1.4. In fact, some uses of email consider the entire email
service, including Author and Recipient, as a subordinate component.
For these services, "end-to-end" refers to points outside the email
service. Examples are voicemail over email [RFC3801], EDI
(Electronic Data Interchange) over email [RFC1767], and facsimile
over email [RFC4142].
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RFC 5598 Email Architecture July 2009
* No requirement for Author, Originator, or Recipients to be
online at the same time
The end-to-end portion of the service is the email object, called a
"message". Broadly, the message itself distinguishes control
information, for handling, from the Author's content.
A precept to the design of mail over the open Internet is permitting
User-to-User and MTA-to-MTA interoperability without prior, direct
arrangement between the independent administrative authorities
responsible for handling a message. All participants rely on having
the core services universally supported and accessible, either
directly or through Gateways that act as translators between Internet
Mail and email environments conforming to other standards. Given the
importance of spontaneity and serendipity in interpersonal
communications, not requiring such prearrangement between
participants is a core benefit of Internet Mail and remains a core
requirement for it.
Within localized networks at the edge of the public Internet, prior
administrative arrangement often is required and can include access
control, routing constraints, and configuration of the information
query service. Although Recipient authentication has usually been
required for message access since the beginning of Internet Mail, in
recent years it also has been required for message submission. In
these cases, a server validates the client's identity, whether by
explicit security protocols or by implicit infrastructure queries to
identify "local" participants.
1.2. The Role of This Architecture
An Internet service is an integration of related capabilities among
two or more participating nodes. The capabilities are accomplished
across the Internet by one or more protocols. What connects a
protocol to a service is an architecture. An architecture specifies
how the protocols implement the service by defining the logical
components of a service and the relationships among them. From that
logical view, a service defines what is being done, an architecture
defines where the pieces are (in relation to each other), and a
protocol defines how particular capabilities are performed.
As such, an architecture will more formally describe a service at a
relatively high level. A protocol that implements some portion of a
service will conform to the architecture to a greater or lesser
extent, depending on the pragmatic tradeoffs they make when trying to
implement the architecture in the context of real-world constraints.
Failure to precisely follow an architecture is not a failure of the
protocol, nor is failure to precisely cast a protocol a failure of
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the architecture. Where a protocol varies from the architecture, it
will of course be appropriate for it to explain the reason for the
variance. However, such variance is not a mark against a protocol:
Happily, the IETF prefers running code to architectural purity.
In this particular case, this architecture attempts to define the
logical components of Internet email and does so post hoc, trying to
capture the architectural principles that the current email protocols
embody. To different extents, email protocols will conform to this
architecture more or less well. Insofar as this architecture differs
from those protocols, the reasons are generally well understood and
are required for interoperation. The differences are not a sign that
protocols need to be fixed. However, this architecture is a best
attempt at a logical model of Internet email, and insofar as new
protocol development varies from this architecture, it is necessary
for designers to understand those differences and explain them
carefully.
1.3. Document Conventions
References to structured fields of a message use a two-part dotted
notation. The first part cites the document that contains the
specification for the field, and the second part is the name of the
field. Hence <RFC5322.From> is the IMF From: header field in an
email content header, and <RFC5321.MailFrom> is the address in the
SMTP "Mail From" command.
When occurring without the IMF (RFC 5322) qualifier, header field
names are shown with a colon suffix. For example, From:.
References to labels for actors, functions or components have the
first letter capitalized.
2. Responsible Actor Roles
Internet Mail is a highly distributed service, with a variety of
Actors playing different roles. These Actors fall into three basic
types:
* User
* Message Handling Service (MHS)
* ADministrative Management Domain (ADMD)
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Although related to a technical architecture, the focus on Actors
concerns participant responsibilities, rather than functionality of
modules. For that reason, the labels used are different from those
used in classic diagrams of email architecture.
2.1. User Actors
Users are the sources and sinks of messages. Users can be people,
organizations, or processes. They can have an exchange that
iterates, and they can expand or contract the set of Users that
participate in a set of exchanges. In Internet Mail, there are four
types of Users:
* Authors
* Recipients
* Return Handlers
* Mediators
Figure 2 shows the primary and secondary flows of messages among
them. As a pragmatic heuristic: User Actors can generate, modify, or
look at the whole message.
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RFC 5598 Email Architecture July 20092.1.2. Recipient
The Recipient is a consumer of the delivered message. The MHS has a
Receiver role (Section 2.2.4) that correlates with the Recipient
role. This is labeled Recv in Figure 3.
Any Recipient can close the user-communication loop by creating and
submitting a new message that replies to the Author. An example of
an automated form of reply is the Message Disposition Notification
(MDN), which informs the Author about the Recipient's handling of the
message. (See Section 4.1.)
2.1.3. Return Handler
Also called "Bounce Handler", the Return Handler is a special form of
Recipient tasked with servicing notifications generated by the MHS as
it transfers or delivers the message. (See Figure 3.) These notices
can be about failures or completions and are sent to an address that
is specified by the Originator. This Return Handling address (also
known as a Return Address) might have no visible characteristics in
common with the address of the Author or Originator.
2.1.4. Mediator
A Mediator receives, aggregates, reformulates, and redistributes
messages among Authors and Recipients who are the principals in
(potentially) protracted exchanges. This activity is easily confused
with the underlying MHS transfer exchanges. However, each serves
very different purposes and operates in very different ways.
When mail is delivered to the Mediator specified in the
RFC5321.RcptTo command for the original message, the MHS handles it
the same way as for any other Recipient. In particular, the MHS sees
each posting and delivery activity between sources and sinks as
independent; it does not see subsequent re-posting as a continuation
of a process. Because the Mediator originates messages, it can
receive replies. Hence, when submitting a reformulated message, the
Mediator is an Author, albeit an Author actually serving as an agent
of one or more other Authors. So a Mediator really is a full-fledged
User. Mediators are considered extensively in Section 5.
A Mediator attempts to preserve the original Author's information in
the message it reformulates but is permitted to make meaningful
changes to the message content or envelope. The MHS sees a new
message, but Users receive a message that they interpret as being
from, or at least initiated by, the Author of the original message.
The role of a Mediator is not limited to merely connecting other
participants; the Mediator is responsible for the new message.
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A Mediator's role is complex and contingent, for example, modifying
and adding content or regulating which Users are allowed to
participate and when. The common example of this role is a group
Mailing List. In a more complex use, a sequence of Mediators could
perform a sequence of formal steps, such as reviewing, modifying, and
approving a purchase request.
A Gateway is a particularly interesting form of Mediator. It is a
hybrid of User and Relay that connects heterogeneous mail services.
Its purpose is to emulate a Relay. For a detailed discussion, see
Section 2.2.3.
2.2. Message Handling Service (MHS) Actors
The Message Handling Service (MHS) performs a single end-to-end
transfer on behalf of the Author to reach the Recipient addresses
specified in the original RFC5321.RcptTo commands. Exchanges that
are either mediated or iterative and protracted, such as those used
for collaboration over time, are handled by the User Actors, not by
the MHS Actors. As a pragmatic heuristic MHS Actors generate,
modify, or look at only transfer data, rather than the entire
message.
Figure 3 shows the relationships among transfer participants in
Internet Mail. Although it shows the Originator (labeled Origin) as
distinct from the Author, and Receiver (labeled Recv) as distinct
from Recipient, each pair of roles usually has the same Actor.
Transfers typically entail one or more Relays. However, direct
delivery from the Originator to Receiver is possible. Intra-
organization mail services usually have only one Relay.
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The Originator also performs any post-submission, Author-related
administrative tasks associated with message transfer and delivery.
Notably, these tasks pertain to sending error and delivery notices,
enforcing local policies, and dealing with messages from the Author
that prove to be problematic for the Internet. The Originator is
accountable for the message content, even when it is not responsible
for it. The Author creates the message, but the Originator handles
any transmission issues with it.
2.2.2. Relay
The Relay performs MHS-level transfer-service routing and store-and-
forward by transmitting or retransmitting the message to its
Recipients. The Relay adds trace information [RFC2505] but does not
modify the envelope information or the message content semantics. It
can modify message content representation, such as changing the form
of transfer encoding from binary to text, but only as required to
meet the capabilities of the next hop in the MHS.
A Message Handling System (MHS) network consists of a set of Relays.
This network is above any underlying packet-switching network that
might be used and below any Gateways or other Mediators.
In other words, email scenarios can involve three distinct
architectural layers, each providing its own type of data of store-
and-forward service:
* User Mediators
* MHS Relays
* Packet Switches
The bottom layer is the Internet's IP service. The most basic email
scenarios involve Relays and Switches.
When a Relay stops attempting to transfer a message, it becomes an
Author because it sends an error message to the Return Address. The
potential for looping is avoided by omitting a Return Address from
this message.
2.2.3. Gateway
A Gateway is a hybrid of User and Relay that connects heterogeneous
mail services. Its purpose is to emulate a Relay and the closer it
comes to this, the better. A Gateway operates as a User when it
needs the ability to modify message content.
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Differences between mail services can be as small as minor syntax
variations, but they usually encompass significant, semantic
distinctions. One difference could be email addresses that are
hierarchical and machine-specific rather than a flat, global
namespace. Another difference could be support for text-only content
or multimedia. Hence the Relay function in a Gateway presents a
significant design challenge if the resulting performance is to be
seen as nearly seamless. The challenge is to ensure User-to-User
functionality between the services, despite differences in their
syntax and semantics.
The basic test of Gateway design is whether an Author on one side of
a Gateway can send a useful message to a Recipient on the other side,
without requiring changes to any components in the Author's or
Recipient's mail services other than adding the Gateway. To each of
these otherwise independent services, the Gateway appears to be a
native participant. But the ultimate test of Gateway design is
whether the Author and Recipient can sustain a dialogue. In
particular, can a Recipient's MUA automatically formulate a valid
Reply that will reach the Author?
2.2.4. Receiver
The Receiver performs final delivery or sends the message to an
alternate address. It can also perform filtering and other policy
enforcement immediately before or after delivery.
2.3. Administrative Actors
Administrative Actors can be associated with different organizations,
each with its own administrative authority. This operational
independence, coupled with the need for interaction between groups,
provides the motivation to distinguish among ADministrative
Management Domains (ADMDs). Each ADMD can have vastly different
operating policies and trust-based decision-making. One obvious
example is the distinction between mail that is exchanged within an
organization and mail that is exchanged between independent
organizations. The rules for handling both types of traffic tend to
be quite different. That difference requires defining the boundaries
of each, and this requires the ADMD construct.
Operation of Internet Mail services is carried out by different
providers (or operators). Each can be an independent ADMD. This
independence of administrative decision-making defines boundaries
that distinguish different portions of the Internet Mail service. A
department that operates a local Relay, an IT department that
operates an enterprise Relay, and an ISP that operates a public
shared email service can be configured into many combinations of
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administrative and operational relationships. Each is a distinct
ADMD, potentially having a complex arrangement of functional
components. Figure 4 depicts relationships among ADMDs. The benefit
of the ADMD construct is that it facilitates discussion about
designs, policies, and operations that need to distinguish between
internal issues and external ones.
The architectural impact of the need for boundaries between ADMDs is
discussed in [Tussle]. Most significant is that the entities
communicating across ADMD boundaries typically have the added burden
of enforcing organizational policies concerning external
communications. At a more mundane level, routing mail between ADMDs
can be an issue, such as needing to route mail between organizational
partners over specially trusted paths.
These are three basic types of ADMDs:
Edge: Independent transfer services in networks at the edge of
the open Internet Mail service.
Consumer: Might be a type of Edge service, as is common for web-
based email access.
Transit: Mail Service Providers (MSPs) that offer value-added
capabilities for Edge ADMDs, such as aggregation and
filtering.
The mail-level transit service is different from packet-level
switching. End-to-end packet transfers usually go through
intermediate routers; email exchange across the open Internet can be
directly between the Boundary MTAs of Edge ADMDs. This distinction
between direct and indirect interaction highlights the differences
discussed in Section 2.2.2.
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RFC 5598 Email Architecture July 2009
These are common examples of ADMDs:
Enterprise Service Providers:
These ADMDs operate the internal data and/or the mail services
within an organization.
Internet Service Providers (ISP):
These ADMDs operate the underlying data communication services,
which are used by one or more Relay and User. ISPs are not
responsible for performing email functions, but they can provide
an environment in which those functions can be performed.
Mail Service Providers:
These ADMDs operate email services, such as for consumers or
client companies.
Practical operational concerns demand that providers be involved in
administration and enforcement issues. This involvement can extend
to operators of lower-level packet services.
3. Identities
The forms of identity used by Internet Mail are: mailbox, domain
name, message-ID, and ENVID (envelope identifier). Each is globally
unique.
3.1. Mailbox
"A mailbox receives mail. It is a conceptual entity that does not
necessarily pertain to file storage." [RFC5322]
A mailbox is specified as an Internet Mail address <addr-spec>. It
has two distinct parts, separated by an at-sign (@). The right side
is a globally interpreted domain name associated with an ADMD.
Domain names are discussed in Section 3.3. Formal Internet Mail
addressing syntax can support source routes to indicate the path
through which a message ought to be sent. The use of source routes
is not common and has been deprecated in [RFC5321].
The portion to the left of the at-sign contains a string that is
globally opaque and is called the <local-part>. It is interpreted
only by the entity specified by the address's domain name. Except as
noted later in this section, all other entities treat the
<local-part> as an uninterpreted literal string and preserve all
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of its original details. As such, its public distribution is
equivalent to sending a Web browser "cookie" that is only interpreted
upon being returned to its creator.
Some local-part values have been standardized for contacting
personnel at an organization. These names cover common operations
and business functions [RFC2142].
It is common for sites to have local structuring conventions for the
left-hand side, <local-part>, of an <addr-spec>. This permits sub-
addressing, such as for distinguishing different discussion groups
used by the same participant. However, it is worth stressing that
these conventions are strictly private to the User's organization and
are not interpreted by any domain except the one listed in the right
side of the <addr-spec>. The exceptions are those specialized
services that conform to public, standardized conventions, as noted
below.
Basic email addressing defines the <local-part> as being globally
opaque. However, there are some uses of email that add a
standardized, global schema to the value, such as between an Author
and a Gateway. The <local-part> details remain invisible to the
public email transfer infrastructure, but provide addressing and
handling instructions for further processing by the Gateway.
Standardized examples of these conventions are the telephone
numbering formats for the Voice Profile for Internet Mail (VPIM)
[RFC3801], such as:
+16137637582@vpim.example.com,
and iFax ([RFC3192], [RFC4143] such as:
FAX=+12027653000/T33S=1387@ifax.example.com.
3.2. Scope of Email Address Use
Email addresses are being used far beyond their original role in
email transfer and delivery. In practical terms, an email address
string has become the common identifier for representing online
identity. Hence, it is essential to be clear about both the nature
and role of an identity string in a particular context and the entity
responsible for setting that string. For example, see Sections
4.1.4, 4.3.3, and 5.
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RFC 5598 Email Architecture July 20093.3. Domain Names
A domain name is a global reference to an Internet resource, such as
a host, a service, or a network. A domain name usually maps to one
or more IP Addresses. Conceptually, the name can encompass an
organization, a collection of machines integrated into a homogeneous
service, or a single machine. A domain name can be administered to
refer to an individual User, but this is not common practice. The
name is structured as a hierarchical sequence of labels, separated by
dots (.), with the top of the hierarchy being on the right end of the
sequence. There can be many names in the sequence -- that is, the
depth of the hierarchy can be substantial. Domain names are defined
and operated through the Domain Name System (DNS) ([RFC1034],
[RFC1035], [RFC2181]).
When not part of a mailbox address, a domain name is used in Internet
Mail to refer to the ADMD or to the host that took action upon the
message, such as providing the administrative scope for a message
identifier or performing transfer processing.
3.4. Message Identifier
There are two standardized tags for identifying messages: Message-ID:
and ENVID. A Message-ID: pertains to content, and an ENVID pertains
to transfer.
3.4.1. Message-ID
IMF provides for, at most, a single Message-ID:. The Message-ID: for
a single message, which is a user-level IMF tag, has a variety of
uses including threading, aiding identification of duplicates, and
DSN (Delivery Status Notification) tracking. The Originator assigns
the Message-ID:. The Recipient's ADMD is the intended consumer of
the Message-ID:, although any Actor along the transfer path can use
it.
Message-ID: is globally unique. Its format is similar to that of a
mailbox, with two distinct parts separated by an at-sign (@).
Typically, the right side specifies the ADMD or host that assigns the
identifier, and the left side contains a string that is globally
opaque and serves to uniquely identify the message within the domain
referenced on the right side. The duration of uniqueness for the
message identifier is undefined.
When a message is revised in any way, the decision whether to assign
a new Message-ID: requires a subjective assessment to determine
whether the editorial content has been changed enough to constitute a
new message. [RFC5322] states that "a message identifier pertains to
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exactly one version of a particular message; subsequent revisions to
the message each receive new message identifiers." Yet experience
suggests that some flexibility is needed. An impossible test is
whether the Recipient will consider the new message to be equivalent
to the old one. For most components of Internet Mail, there is no
way to predict a specific Recipient's preferences on this matter.
Both creating and failing to create a new Message-ID: have their
downsides.
Here are some guidelines and examples:
o If a message is changed only in form, such as character encoding,
it is still the same message.
o If a message has minor additions to the content, such as a Mailing
List tag at the beginning of the RFC5322.Subject header field, or
some Mailing List administrative information added to the end of
the primary body part text, it is probably the same message.
o If a message has viruses deleted from it, it is probably the same
message.
o If a message has offensive words deleted from it, some Recipients
will consider it the same message, but some will not.
o If a message is translated into a different language, some
Recipients will consider it the same message, but some will not.
o If a message is included in a digest of messages, the digest
constitutes a new message.
o If a message is forwarded by a Recipient, what is forwarded is a
new message.
o If a message is "redirected", such as using IMF "Resent-*" header
fields, some Recipients will consider it the same message, but
some will not.
The absence of both objective, precise criteria for regenerating a
Message-ID: and strong protection associated with the string means
that the presence of an ID can permit an assessment that is
marginally better than a heuristic, but the ID certainly has no value
on its own for strict formal reference or comparison. For that
reason, the Message-ID: is not intended to be used for any function
that has security implications.
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RFC 5598 Email Architecture July 20093.4.2. ENVID
The ENVID (envelope identifier) can be used for message-tracking
purposes ([RFC3885], [RFC3464]) concerning a single posting/delivery
transfer. The ENVID labels a single transit of the MHS by a specific
message. So, the ENVID is used for one message posting until that
message is delivered. A re-posting of the message, such as by a
Mediator, does not reuse that ENVID, but can use a new one, even
though the message might legitimately retain its original
Message-ID:.
The format of an ENVID is free form. Although its creator might
choose to impose structure on the string, none is imposed by Internet
standards. By implication, the scope of the string is defined by the
domain name of the Return Address.
4. Services and Standards
The Internet Mail architecture comprises six basic types of
functionality, which are arranged to support a store-and-forward
service. As shown in Figure 5, each type can have multiple
instances, some of which represent specialized roles. This section
considers the activities and relationships among these components,
and the Internet Mail standards that apply to them.
Message
Message User Agent (MUA)
Author MUA (aMUA)
Recipient MUA (rMUA)
Message Submission Agent (MSA)
Author-focused MSA functions (aMSA)
MHS-focused MSA functions (hMSA)
Message Transfer Agent (MTA)
Message Delivery Agent (MDA)
Recipient-focused MDA functions (rMDA)
MHS-focused MDA functions (hMDA)
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... lines indicate supporting transfers or roles
*** lines indicate aggregated service
Figure 5: Protocols and Services
4.1. Message Data
The purpose of the Message Handling System (MHS) is to exchange an
IMF message object among participants [RFC5322]. All of its
underlying mechanisms serve to deliver that message from its Author
to its Recipients. A message can be explicitly labeled as to its
nature [RFC3458].
A message comprises a transit-handling envelope and the message
content. The envelope contains information used by the MHS. The
content is divided into a structured header and the body. The header
comprises transit-handling trace information and structured fields
that are part of the Author's message content. The body can be
unstructured lines of text or a tree of multimedia subordinate
objects, called "body-parts" or, popularly, "attachments".
[RFC2045], [RFC2046], [RFC2047], [RFC4288], [RFC4289], [RFC2049].
In addition, Internet Mail has a few conventions for special control
data, notably:
Delivery Status Notification (DSN):
A Delivery Status Notification (DSN) is a message that can be
generated by the MHS (MSA, MTA, or MDA) and sent to the
RFC5321.MailFrom address. MDA and MTA are shown as sources of
DSNs in Figure 5, and the destination is shown as Returns. DSNs
provide information about message transit, such as transfer errors
or successful delivery [RFC3461].
Message Disposition Notification (MDN):
A Message Disposition Notification (MDN) is a message that
provides information about post-delivery processing, such as
indicating that the message has been displayed [RFC3798] or the
form of content that can be supported [RFC3297]. It can be
generated by an rMUA and is sent to the
Disposition-Notification-To addresses. The mailbox for this is
shown as Disp in Figure 5.
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Message Filtering (SIEVE):
Sieve is a scripting language used to specify conditions for
differential handling of mail, typically at the time of delivery
[RFC5228]. Scripts can be conveyed in a variety of ways, such as
a MIME part in a message. Figure 5 shows a Sieve script going
from the rMUA to the MDA. However, filtering can be done at many
different points along the transit path, and any one or more of
them might be subject to Sieve directives, especially within a
single ADMD. Figure 5 shows only one relationship, for (relative)
simplicity.
4.1.1. Envelope
Internet Mail has a fragmented framework for transit-related handling
information. Information that is used directly by the MHS is called
the "envelope". It directs handling activities by the transfer
service and is carried in transfer-service commands. That is, the
envelope exists in the transfer protocol SMTP [RFC5321].
Trace information, such as RFC5322.Received, is recorded in the
message header and is not subsequently altered [RFC5322].
4.1.2. Header Fields
Header fields are attribute name/value pairs that cover an extensible
range of email-service parameters, structured user content, and user
transaction meta-information. The core set of header fields is
defined in [RFC5322]. It is common practice to extend this set for
different applications. Procedures for registering header fields are
defined in [RFC3864]. An extensive set of existing header field
registrations is provided in [RFC4021].
One danger of placing additional information in header fields is that
Gateways often alter or delete them.
4.1.3. Body
The body of a message might be lines of ASCII text or a
hierarchically structured composition of multimedia body part
attachments using MIME ([RFC2045], [RFC2046], [RFC2047], [RFC4288],
and [RFC2049]).
4.1.4. Identity References in a Message
Table 1 lists the core identifiers present in a message during
transit.
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RFC 5598 Email Architecture July 2009RFC5322.Reply-To: Set by - Author
If a Recipient sends a reply message that would otherwise use the
RFC5322.From field addresses in the original message, the
addresses in the RFC5322.Reply-To field are used instead. In
other words, this field overrides the From: field for responses
from Recipients.
RFC5322.Sender: Set by - Originator
This field specifies the address responsible for submitting the
message to the transfer service. This field can be omitted if it
contains the same address as RFC5322.From. However, omitting this
field does not mean that no Sender is specified; it means that
that header field is virtual and that the address in the From:
field is to be used.
Specification of the notifications Return Addresses, which are
contained in RFC5321.MailFrom, is made by the RFC5322.Sender.
Typically, the Return address is the same as the Sender address.
However, some usage scenarios require it to be different.
RFC5322.To/.CC: Set by - Author
These fields specify MUA Recipient addresses. However, some or
all of the addresses in these fields might not be present in the
RFC5321.RcptTo commands.
The distinction between To and CC is subjective. Generally, a To
addressee is considered primary and is expected to take action on
the message. A CC addressee typically receives a copy as a
courtesy.
RFC5322.BCC: Set by - Author
A copy of the message might be sent to an addressee whose
participation is not to be disclosed to the RFC5322.To or
RFC5322.CC Recipients and, usually, not to the other BCC
Recipients. The BCC: header field indicates a message copy to
such a Recipient. Use of this field is discussed in [RFC5322].
RFC5321.HELO/.EHLO: Set by - Originator, MSA, MTA
Any SMTP client -- including Originator, MSA, or MTA -- can
specify its hosting domain identity for the SMTP HELO or EHLO
command operation.
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RFC 5598 Email Architecture July 2009RFC3461.ENVID: Set by - Originator
The MSA can specify an opaque string, to be included in a DSN, as
a means of assisting the Return Address Recipient in identifying
the message that produced a DSN or message tracking.
RFC5321.MailFrom: Set by - Originator
This field is an end-to-end string that specifies an email address
for receiving return control information, such as returned
messages. The name of this field is misleading, because it is not
required to specify either the Author or the Actor responsible for
submitting the message. Rather, the Actor responsible for
submission specifies the RFC5321.MailFrom address. Ultimately,
the simple basis for deciding which address needs to be in the
RFC5321.MailFrom field is to determine which address is to be
informed about transfer-level problems (and possibly successes).
RFC5321.RcptTo: Set by - Author, Final MTA, MDA
This field specifies the MUA mailbox address of a Recipient. The
string might not be visible in the message content header. For
example, the IMF destination address header fields, such as
RFC5322.To, might specify a Mailing List mailbox, while the
RFC5321.RcptTo address specifies a member of that list.
RFC5321.ORCPT: Set by - Originator.
This is an optional parameter to the RCPT command, indicating the
original address to which the current RCPT TO address corresponds,
after a mapping was performed during transit. An ORCPT is the
only reliable way to correlate a DSN from a multi-Recipient
message transfer with the intended Recipient.
RFC5321.Received: Set by - Originator, Relay, Mediator, Dest
This field contains trace information, including originating host,
Relays, Mediators, and MSA host domain names and/or IP Addresses.
RFC5321.Return-Path: Set by - Originator
The MDA records the RFC5321.MailFrom address into the
RFC5321.Return-Path field.
RFC2919.List-Id: Set by - Mediator, Author
This field provides a globally unique Mailing List naming
framework that is independent of particular hosts [RFC2919].
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The identifier is in the form of a domain name; however, the
string usually is constructed by combining the two parts of an
email address. The result is rarely a true domain name, listed in
the domain name service, although it can be.
RFC2369.List-*: Set by - Mediator, Author
[RFC2369] defines a collection of message header fields for use by
Mailing Lists. In effect, they supply list-specific parameters
for common Mailing-List user operations. The identifiers for
these operations are for the list itself and the user-as-
subscriber [RFC2369].
RFC0791.SourceAddr: Set by - The Client SMTP sending host
immediately preceding the current receiving SMTP server
[RFC0791] defines the basic unit of data transfer for the
Internet: the IP datagram. It contains a Source Address field
that specifies the IP Address for the host (interface) from which
the datagram was sent. This information is set and provided by
the IP layer, which makes it independent of mail-level mechanisms.
As such, it is often taken to be authoritative, although it is
possible to provide false addresses.
4.2. User-Level Services
Interactions at the user level entail protocol exchanges, distinct
from those that occur at lower layers of the Internet Mail MHS
architecture that is, in turn, above the Internet Transport layer.
Because the motivation for email, and much of its use, is for
interaction among people, the nature and details of these protocol
exchanges often are determined by the needs of interpersonal and
group communication. To accommodate the idiosyncratic behavior
inherent in such communication, only subjective guidelines, rather
than strict rules, can be offered for some aspects of system
behavior. Mailing Lists provide particularly salient examples.
4.2.1. Message User Agent (MUA)
A Message User Agent (MUA) works on behalf of User Actors and User
applications. It is their representative within the email service.
The Author MUA (aMUA) creates a message and performs initial
submission into the transfer infrastructure via a Mail Submission
Agent (MSA). It can also perform any creation- and posting-time
archiving in its Message Store (aMS). An MUA aMS can organize
messages in many different ways. A common model uses aggregations,
called "folders"; in IMAP they are called "mailboxes". This model
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allows a folder for messages under development (Drafts), a folder for
messages waiting to be sent (Queued or Unsent), and a folder for
messages that have been successfully posted for transfer (Sent). But
none of these folders is required. For example, IMAP allows drafts
to be stored in any folder, so no Drafts folder needs to be present.
The Recipient MUA (rMUA) works on behalf of the Recipient to process
received mail. This processing includes generating user-level
disposition control messages, displaying and disposing of the
received message, and closing or expanding the user-communication
loop by initiating replies and forwarding new messages.
NOTE: Although not shown in Figure 5, an MUA itself can have a
distributed implementation, such as a "thin" user-interface
module on a constrained device such as a smartphone, with
most of the MUA functionality running remotely on a more
capable server. An example of such an architecture might use
IMAP [RFC3501] for most of the interactions between an MUA
client and an MUA server. An approach for such scenarios is
defined by [RFC4550].
A Mediator is a special class of MUA. It performs message
re-posting, as discussed in Section 2.1.
An MUA can be automated, on behalf of a User who is not present at
the time the MUA is active. One example is a bulk sending service
that has a timed-initiation feature. These services are not to be
confused with a Mailing List Mediator, since there is no incoming
message triggering the activity of the automated service.
A popular and problematic MUA is an automatic responder, such as one
that sends out-of-office notices. This behavior might be confused
with that of a Mediator, but this MUA is generating a new message.
Automatic responders can annoy Users of Mailing Lists unless they
follow [RFC3834].
The identity fields are relevant to a typical MUA:
RFC5322.From
RFC5322.Reply-To
RFC5322.Sender
RFC5322.To, RFC5322.CC
RFC5322.BCC
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RFC 5598 Email Architecture July 20094.2.2. Message Store (MS)
An MUA can employ a long-term Message Store (MS). Figure 5 depicts
an Author's MS (aMS) and a Recipient's MS (rMS). An MS can be
located on a remote server or on the same machine as the MUA.
An MS acquires messages from an MDA either proactively by a local
mechanism or even by a standardized mechanism such as SMTP(!), or
reactively by using POP or IMAP. The MUA accesses the MS either by a
local mechanism or by using POP or IMAP. Using POP for individual
message accesses, rather than for bulk transfer, is relatively rare
and inefficient.
4.3. MHS-Level Services4.3.1. Mail Submission Agent (MSA)
A Mail Submission Agent (MSA) accepts the message submitted by the
aMUA and enforces the policies of the hosting ADMD and the
requirements of Internet standards. An MSA represents an unusual
functional dichotomy. It represents the interests of the Author
(aMUA) during message posting, to facilitate posting success; it also
represents the interests of the MHS. In the architecture, these
responsibilities are modeled, as shown in Figure 5, by dividing the
MSA into two sub-components, aMSA and hMSA, respectively. Transfer
of responsibility for a single message, from an Author's environment
to the MHS, is called "posting". In Figure 5, it is marked as the
(S) transition, within the MSA.
The hMSA takes transit responsibility for a message that conforms to
the relevant Internet standards and to local site policies. It
rejects messages that are not in conformance. The MSA performs final
message preparation for submission and effects the transfer of
responsibility to the MHS, via the hMSA. The amount of preparation
depends upon the local implementations. Examples of aMSA tasks
include adding header fields, such as Date: and Message-ID:, and
modifying portions of the message from local notations to Internet
standards, such as expanding an address to its formal IMF
representation.
Historically, standards-based MUA/MSA message postings have used SMTP
[RFC5321]. The standard currently preferred is SUBMISSION [RFC4409].
Although SUBMISSION derives from SMTP, it uses a separate TCP port
and imposes distinct requirements, such as access authorization.
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These identities are relevant to the MSA:
RFC5321.HELO/.EHLO
RFC3461.ENVID
RFC5321.MailFrom
RFC5321.RcptTo
RFC5321.Received
RFC0791.SourceAddr
4.3.2. Message Transfer Agent (MTA)
A Message Transfer Agent (MTA) relays mail for one application-level
"hop". It is like a packet switch or IP router in that its job is to
make routing assessments and to move the message closer to the
Recipients. Of course, email objects are typically much larger than
the payload of a packet or datagram, and the end-to-end latencies are
typically much higher. Relaying is performed by a sequence of MTAs
until the message reaches a destination MDA. Hence, an MTA
implements both client and server MTA functionality; it does not
change addresses in the envelope or reformulate the editorial
content. A change in data form, such as to MIME Content-Transfer-
Encoding, is within the purview of an MTA, but removal or replacement
of body content is not. An MTA also adds trace information
[RFC2505].
NOTE: Within a destination ADMD, email-relaying modules can make a
variety of changes to the message, prior to delivery. In
such cases, these modules are acting as Gateways, rather than
MTAs.
Internet Mail uses SMTP ([RFC5321], [RFC2821], [RFC0821]) primarily
to effect point-to-point transfers between peer MTAs. Other transfer
mechanisms include Batch SMTP [RFC2442] and On-Demand Mail Relay
(ODMR) SMTP [RFC2645]. As with most network-layer mechanisms, the
Internet Mail SMTP supports a basic level of reliability, by virtue
of providing for retransmission after a temporary transfer failure.
Unlike typical packet switches (and Instant Messaging services),
Internet Mail MTAs are expected to store messages in a manner that
allows recovery across service interruptions, such as host-system
shutdown. The degree of such robustness and persistence by an MTA
can vary. The base SMTP specification provides a framework for
protocol response codes. An extensible enhancement to this framework
is defined in [RFC5248].
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Although quite basic, the dominant routing mechanism for Internet
Mail is the DNS MX record [RFC1035], which specifies an MTA through
which the queried domain can be reached. This mechanism presumes a
public, or at least a common, backbone that permits any attached MTA
to connect to any other.
MTAs can perform any of these well-established roles:
Boundary MTA: An MTA that is part of an ADMD and interacts with MTAs
in other ADMDs. This is also called a Border MTA.
There can be different Boundary MTAs, according to the
direction of mail-flow.
Outbound MTA: An MTA that relays messages to other
ADMDs.
Inbound MTA: An MTA that receives inbound SMTP
messages from MTA Relays in other
ADMDs, for example, an MTA running on
the host listed as the target of an MX
record.
Final MTA: The MTA that transfers a message to the MDA.
These identities are relevant to the MTA:
RFC5321.HELO/.EHLO
RFC3461.ENVID
RFC5321.MailFrom
RFC5321.RcptTo
RFC5322.Received: Set by - Relay Server
RFC0791.SourceAddr
4.3.3. Mail Delivery Agent (MDA)
A transfer of responsibility from the MHS to a Recipient's
environment (mailbox) is called "delivery". In the architecture, as
depicted in Figure 5, delivery takes place within a Mail Delivery
Agent (MDA) and is shown as the (D) transition from the MHS-oriented
MDA component (hMDA) to the Recipient-oriented MDA component (rMDA).
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An MDA can provide distinctive, address-based functionality, made
possible by its detailed information about the properties of the
destination address. This information might also be present
elsewhere in the Recipient's ADMD, such as at an organizational
border (Boundary) Relay. However, it is required for the MDA, if
only because the MDA is required to know where to deliver the
message.
Like an MSA, an MDA serves two roles, as depicted in Figure 5.
Formal transfer of responsibility, called "delivery", is effected
between the two components that embody these roles and is shown as
"(D)" in Figure 5. The MHS portion (hMDA) primarily functions as a
server SMTP engine. A common additional role is to redirect the
message to an alternative address, as specified by the Recipient
addressee's preferences. The job of the Recipient portion of the MDA
(rMDA) is to perform any delivery actions that the Recipient
specifies.
Transfer into the MDA is accomplished by a normal MTA transfer
mechanism. Transfer from an MDA to an MS uses an access protocol,
such as POP or IMAP.
NOTE: The term "delivery" can refer to the formal, MHS function
specified here or to the first time a message is displayed to
a Recipient. A simple, practical test for whether the MHS-
based definition applies is whether a DSN can be generated.
These identities are relevant to the MDA:
RFC5321.Return-Path: Set by - Author Originator or Mediator
Originator
The MDA records the RFC5321.MailFrom address into the
RFC5321.Return-Path field.
RFC5322.Received: Set by - MDA server
An MDA can record a Received: header field to indicate trace
information, including source host and receiving host domain
names and/or IP Addresses.
4.4. Transition Modes
From the origination site to the point of delivery, Internet Mail
usually follows a "push" model. That is, the Actor that holds the
message initiates transfer to the next venue, typically with SMTP
[RFC5321] or the Local Mail Transfer Protocol (LMTP) [RFC2033]. With
a "pull" model, the Actor that holds the message waits for the Actor
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RFC 5598 Email Architecture July 2009
in the next venue to initiate a request for transfer. Standardized
mechanisms for pull-based MHS transfer are ETRN [RFC1985] and ODMR
[RFC2645].
After delivery, the Recipient's MUA (or MS) can gain access by having
the message pushed to it or by having the receiver of access pull the
message, such as by using POP [RFC1939] and IMAP [RFC3501].
4.5. Implementation and Operation
A discussion of any interesting system architecture often bogs down
when architecture and implementation are confused. An architecture
defines the conceptual functions of a service, divided into discrete
conceptual modules. An implementation of that architecture can
combine or separate architectural components, as needed for a
particular operational environment. For example, a software system
that primarily performs message relaying is an MTA, yet it might also
include MDA functionality. That same MTA system might be able to
interface with non-Internet email services and thus perform both as
an MTA and as a Gateway.
Similarly, implemented modules might be configured to form
elaborations of the architecture. An interesting example is a
distributed MS. One portion might be a remote server and another
might be local to the MUA. As discussed in [RFC1733], there are
three operational relationships among such MSs:
Online: The MS is remote, and messages are accessible only when the
MUA is attached to the MS so that the MUA will re-fetch all or
part of a message from one session to the next.
Offline: The MS is local to the User, and messages are completely
moved from any remote store, rather than (also) being retained
there.
Disconnected: An rMS and a uMS are kept synchronized, for all or
part of their contents, while they are connected. When they are
disconnected, mail can arrive at the rMS and the User can make
changes to the uMS. The two stores are re-synchronized when they
are reconnected.
5. Mediators
Basic message transfer from Author to Recipients is accomplished by
using an asynchronous store-and-forward communication infrastructure
in a sequence of independent transmissions through some number of
MTAs. A very different task is a sequence of postings and deliveries
through Mediators. A Mediator forwards a message through a
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re-posting process. The Mediator shares some functionality with
basic MTA relaying, but has greater flexibility in both addressing
and content than is available to MTAs.
This is the core set of message information that is commonly set by
all types of Mediators:
RFC5321.HELO/.EHLO: Set by - Mediator Originator
RFC3461.ENVID: Set by - Mediator Originator
RFC5321.RcptTo: Set by - Mediator Author
RFC5321.Received: Set by - Mediator Dest
The Mediator can record received information to indicate the
delivery to the original address and submission to the alias
address. The trace of Received: header fields can include
everything from original posting, through relaying, to final
delivery.
The aspect of a Mediator that distinguishes it from any other MUA
creating a message is that a Mediator preserves the integrity and
tone of the original message, including the essential aspects of its
origination information. The Mediator might also add commentary.
Examples of MUA messages that a Mediator does not create include:
New message that forwards an existing message:
Although this action provides a basic template for a class of
Mediators, its typical occurrence is not, itself, an example of
a Mediator. The new message is viewed as being from the Actor
that is doing the forwarding, rather than from the original
Author.
A new message encapsulates the original message and is seen as
from the new Originator. This Mediator Originator might add
commentary and can modify the original message content.
Because the forwarded message is a component of the message
sent by the new Originator, the new message creates a new
dialogue. However, the final Recipient still sees the
contained message as from the original Author.
Reply:
When a Recipient responds to the Author of a message, the new
message is not typically viewed as a forwarding of the
original. Its focus is the new content, although it might
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contain all or part of the material from the original message.
The earlier material is merely contextual and secondary. This
includes automated replies, such as vacation out-of-office
notices, as discussed in Section 4.2.1.
Annotation:
The integrity of the original message is usually preserved, but
one or more comments about the message are added in a manner
that distinguishes commentary from original text. The primary
purpose of the new message is to provide commentary from a new
Author, similar to a Reply.
The remainder of this section describes common examples of Mediators.
5.1. Alias
One function of an MDA is to determine the internal location of a
mailbox in order to perform delivery. An Alias is a simple
re-addressing facility that provides one or more new Internet Mail
addresses, rather than a single, internal one; the message continues
through the transfer service, for delivery to one or more alternate
addresses. Although typically implemented as part of an MDA, this
facility is a Recipient function. It resubmits the message, although
all handling information except the envelope Recipient
(rfc5321.RcptTo) address is retained. In particular, the Return
Address (rfc5321.MailFrom) is unchanged.
What is distinctive about this forwarding mechanism is how closely it
resembles normal MTA store-and-forward relaying. Its only
significant difference is that it changes the RFC5321.RcptTo value.
Because this change is so small, aliasing can be viewed as a part of
the lower-level mail-relaying activity. However, this small change
has a large semantic impact: The designated Recipient has chosen a
new Recipient.
NOTE: When the replacement list includes more than one address, the
alias is increasingly likely to have delivery problems. Any
problem reports go to the original Author, not the
administrator of the alias entry. This makes it more
difficult to resolve the problem, because the original Author
has no knowledge of the Alias mechanism.
Including the core set of message information listed at the beginning
of this section, Alias typically changes:
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RFC 5598 Email Architecture July 2009RFC5322.To/.CC/.BCC: Set by - Author
These fields retain their original addresses.
RFC5321.MailFrom: Set by - Author
The benefit of retaining the original MailFrom value is to
ensure that an Actor related to the originating ADMD knows
there has been a delivery problem. On the other hand, the
responsibility for handling problems, when transiting from the
original Recipient mailbox to the alias mailbox usually lies
with that original Recipient, because the Alias mechanism is
strictly under that Recipient's control. Retaining the
original MailFrom address prevents this.
5.2. ReSender
Also called the ReDirector, the ReSender's actions differ from
forwarding because the Mediator "splices" a message's addressing
information to connect the Author of the original message with the
Recipient of the new message. This connection permits them to have
direct exchange, using their normal MUA Reply functions, while also
recording full reference information about the Recipient who served
as a Mediator. Hence, the new Recipient sees the message as being
from the original Author, even if the Mediator adds commentary.
Including the core set of message information listed at the beginning
of this section, these identities are relevant to a resent message:
RFC5322.From: Set by - original Author
Names and addresses for the original Author of the message
content are retained. The free-form (display-name) portion of
the address might be modified to provide an informal reference
to the ReSender.
RFC5322.Reply-To: Set by - original Author
If this field is present in the original message, it is
retained in the resent message.
RFC5322.Sender: Set by - Author's Originator or Mediator
Originator
RFC5322.To/.CC/.BCC: Set by - original Author
These fields specify the original message Recipients.
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RFC 5598 Email Architecture July 2009RFC5322.Resent-From: Set by - Mediator Author
This address is of the original Recipient who is redirecting
the message. Otherwise, the same rules apply to the Resent-
From: field as to an original RFC5322.From field.
RFC5322.Resent-Sender: Set by - Mediator Originator
The address of the Actor responsible for resubmitting the
message. As with RFC5322.Sender, this field can be omitted
when it contains the same address as RFC5322.Resent-From.
RFC5322.Resent-To/-CC/-BCC: Set by - Mediator Author
The addresses of the new Recipients who are now able to reply
to the original Author.
RFC5321.MailFrom: Set by - Mediator Originator
The Actor responsible for resubmission (RFC5322.Resent-Sender)
is also responsible for specifying the new MailFrom address.
5.3. Mailing Lists
A Mailing List receives messages as an explicit addressee and then
re-posts them to a list of subscribed members. The Mailing List
performs a task that can be viewed as an elaboration of the ReSender.
In addition to sending the new message to a potentially large number
of new Recipients, the Mailing List can modify content, for example,
by deleting attachments, converting the format, and adding list-
specific comments. Mailing Lists also archive messages posted by
Authors. Still the message retains characteristics of being from the
original Author.
Including the core set of message information listed at the beginning
of this section, these identities are relevant to a Mailing List
processor when submitting a message:
RFC2919.List-Id: Set by - Mediator Author
RFC2369.List-*: Set by - Mediator Author
RFC5322.From: Set by - original Author
Names and email addresses for the original Author of the
message content are retained.
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RFC 5598 Email Architecture July 2009RFC5322.Reply-To: Set by - Mediator or original Author
Although problematic, it is common for a Mailing List to assign
its own addresses to the Reply-To: header field of messages
that it posts. This assignment is intended to ensure that
replies go to all list members, rather than to only the
original Author. As a User Actor, a Mailing List is the Author
of the new message and can legitimately set the Reply-To:
value. As a Mediator attempting to represent the message on
behalf of its original Author, creating or modifying a
Reply-To: field can be viewed as violating that Author's
intent. When the Reply-To is modified in this way, a reply
that is meant only for the original Author will instead go to
the entire list. When the Mailing List does not set the field,
a reply meant for the entire list can instead go only to the
original Author. At best, either choice is a matter of group
culture for the particular list.
RFC5322.Sender: Set by - Author Originator or Mediator Originator
This field usually specifies the address of the Actor
responsible for Mailing List operations. Mailing Lists that
operate in a manner similar to a simple MTA Relay preserve as
much of the original handling information as possible,
including the original RFC5322.Sender field. (Note that this
mode of operation causes the Mailing List to behave much like
an Alias, with a possible difference in number of new
addressees.)
RFC5322.To/.CC: Set by - original Author
These fields usually contain the original list of Recipient
addresses.
RFC5321.MailFrom: Set by - Mediator Originator
Because a Mailing List can modify the content of a message in
any way, it is responsible for that content; that is, it is an
Author. As such, the Return Address is specified by the
Mailing List. Although it is plausible for the Mailing List to
reuse the Return Address employed by the original Originator,
notifications sent to that address after a message has been
processed by a Mailing List could be problematic.
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RFC 5598 Email Architecture July 20095.4. Gateways
A Gateway performs the basic routing and transfer work of message
relaying, but it also is permitted to modify content, structure,
address, or attributes as needed to send the message into a messaging
environment that operates under different standards or potentially
incompatible policies. When a Gateway connects two differing
messaging services, its role is easy to identify and understand.
When it connects environments that follow similar technical
standards, but significantly different administrative policies, it is
easy to view a Gateway as merely an MTA.
The critical distinction between an MTA and a Gateway is that a
Gateway can make substantive changes to a message to map between the
standards. In virtually all cases, this mapping results in some
degree of semantic loss. The challenge of Gateway design is to
minimize this loss. Standardized Gateways to Internet Mail are
facsimile [RFC4143], voicemail [RFC3801], and the Multimedia
Messaging Service (MMS) [RFC4356].
A Gateway can set any identity field available to an MUA. Including
the core set of message information listed at the beginning of this
section, these identities are typically relevant to Gateways:
RFC5322.From: Set by - original Author
Names and addresses for the original Author of the message
content are retained. As for all original addressing
information in the message, the Gateway can translate addresses
as required to continue to be useful in the target environment.
RFC5322.Reply-To: Set by - original Author
It is best for a Gateway to retain this information, if it is
present. The ability to perform a successful reply by a
Recipient is a typical test of Gateway functionality.
RFC5322.Sender: Set by - Author Originator or Mediator Originator
This field can retain the original value or can be set to a new
address.
RFC5322.To/.CC/.BCC: Set by - original Recipient
These fields usually retain their original addresses.
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RFC 5598 Email Architecture July 2009RFC5321.MailFrom: Set by - Author Originator or Mediator
Originator
The Actor responsible for handling the message can specify a
new address to receive handling notices.
5.5. Boundary Filter
To enforce security boundaries, organizations can subject messages to
analysis for conformance with its safety policies. An example is
detection of content classed as spam or a virus. A filter might
alter the content to render it safe, such as by removing content
deemed unacceptable. Typically, these actions add content to the
message that records the actions.
6. Considerations6.1. Security Considerations
This document describes the existing Internet Mail architecture. It
introduces no new capabilities. The security considerations of this
deployed architecture are documented extensively in the technical
specifications referenced by this document. These specifications
cover classic security topics, such as authentication and privacy.
For example, email-transfer protocols can use standardized mechanisms
for operation over authenticated and/or encrypted links, and message
content has similar protection standards available. Examples of such
mechanisms include SMTP-TLS [RFC3207], SMTP-Auth [RFC4954], OpenPGP
[RFC4880], and S/MIME [RFC3851].
The core of the Internet Mail architecture does not impose any
security requirements or functions on the end-to-end or hop-by-hop
components. For example, it does not require participant
authentication and does not attempt to prevent data disclosure.
Particular message attributes might expose specific security
considerations. For example, the blind carbon copy feature of the
architecture invites disclosure concerns, as discussed in Section 7.2
of [RFC5321] and Section 5 of [RFC5322]. Transport of text or non-
text content in this architecture has security considerations that
are discussed in [RFC5322], [RFC2045], [RFC2046], and [RFC4288];
also, security considerations are present for some of the media types
registered with IANA.
Agents that automatically respond to email raise significant security
considerations, as discussed in [RFC3834]. Gateway behaviors affect
end-to-end security services, as discussed in [RFC2480]. Security
considerations for boundary filters are discussed in [RFC5228].
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RFC 5598 Email Architecture July 2009
See Section 7.1 of [RFC5321] for a discussion of the topic of
origination validation. As mentioned in Section 4.1.4, it is common
practice for components of this architecture to use the
RFC0791.SourceAddr to make policy decisions [RFC2505], although the
address can be "spoofed". It is possible to use it without
authorization. SMTP and Submission authentication ([RFC4409],
[RFC4954]) provide more secure alternatives.
The discussion of trust boundaries, ADMDs, Actors, roles, and
responsibilities in this document highlights the relevance and
potential complexity of security factors for operation of an Internet
Mail service. The core design of Internet Mail to encourage open and
casual exchange of messages has met with scaling challenges, as the
population of email participants has grown to include those with
problematic practices. For example, spam, as defined in [RFC2505],
is a by-product of this architecture. A number of Standards Track or
BCP documents on the subject have been issued (see [RFC2505],
[RFC5068], and [RFC5235]).
6.2. Internationalization
The core Internet email standards are based on the use of US-ASCII --
that is, SMTP [RFC5321] and IMF [RFC5322], as well as their
predecessors. They describe the transport and composition of
messages as composed strictly of US-ASCII 7-bit encoded characters.
The standards have been incrementally enhanced to allow for
characters outside of this limited set, while retaining mechanisms
for backwards-compatibility. Specifically:
o The MIME specifications ([RFC2045], [RFC2046], [RFC2047],
[RFC2049]) allow for the use of coded character sets and
character-encoding schemes ("charsets" in MIME terminology) other
than US-ASCII. MIME's [RFC2046] allows the textual content of a
message to have a label affixed that specifies the charset used in
that content. Equally, MIME's [RFC2047] allows the textual
content of certain header fields in a message to be similarly
labeled. However, since messages might be transported over SMTP
implementations only capable of transporting 7-bit encoded
characters, MIME's [RFC2045] also provides for "content transfer
encoding" so that characters of other charsets can be re-encoded
as an overlay to US-ASCII.
o MIME's [RFC2045] allows for the textual content of a message to be
in an 8-bit character-encoding scheme. In order to transport
these without re-encoding them, the SMTP specification supports an
option [RFC1652] that permits the transport of such textual
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RFC 5598 Email Architecture July 2009
content. However, the [RFC1652] option does not address the use
of 8-bit content in message header fields, and therefore [RFC2047]
encoding is still required for those.
o A series of experimental protocols on Email Address
Internationalization (EAI) have been released that extend SMTP and
IMF to allow for 8-bit encoded characters to appear in addresses
and other information throughout the header fields of messages.
[RFC5335] specifies the format of such message header fields
(which encode the characters in UTF-8), and [RFC5336] specifies an
SMTP option for the transport of these messages.
o MIME's [RFC2045] and [RFC2046] allow for the transport of true
multimedia material; such material enables internationalization
because it is not restricted to any particular language or locale.
o The formats for Delivery Status Notifications (DSNs -- [RFC3462],
[RFC3463], [RFC3464]) and Message Disposition Notifications (MDNs
-- [RFC3798]) include both a structured and unstructured
representation of the notification. In the event that the
unstructured representation is in the wrong language or is
otherwise unsuitable for use, this allows an MUA to construct its
own appropriately localized representation of notification for
display to the User.
o POP and IMAP have no difficulties with handling MIME messages,
including ones containing 8bit, and therefore are not a source of
internationalization issues.
Hence, the use of UTF-8 is fully established in existing Internet
Mail. However, support for long-standing encoding forms is retained
and is still used.
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RFC 5598 Email Architecture July 2009Appendix A. Acknowledgments
This work began in 2004 and has evolved through numerous rounds of
community review; it derives from a section in an early version of
[RFC5068]. Over its 5 years of development, the document has gone
through 14 incremental versions, with vigorous community review that
produced many substantive changes. Review was performed in the IETF
and other email technical venues. Although not a formal activity of
the IETF, issues with the document's contents were resolved using the
classic style of IETF community open, group decision-making. The
document is already cited in other work, such as in IMAP and Sieve
specifications and in academic classwork. The step of standardizing
is useful to provide a solid and stable reference to the Internet's
now-complex email service.
Details of the Originator Actor role was greatly clarified during
discussions in the IETF's Marid working group.
Graham Klyne, Pete Resnick, and Steve Atkins provided thoughtful
insight on the framework and details of the original drafts, as did
Chris Newman for the final versions, while also serving as cognizant
Area Director for the document. Tony Hansen served as document
shepherd through the IETF process.
Later reviews and suggestions were provided by Eric Allman, Nathaniel
Borenstein, Ed Bradford, Cyrus Daboo, Frank Ellermann, Tony Finch,
Ned Freed, Eric Hall, Willemien Hoogendoorn, Brad Knowles, John
Leslie, Bruce Valdis Kletnieks, Mark E. Mallett, David MacQuigg,
Alexey Melnikov, der Mouse, S. Moonesamy, Daryl Odnert, Rahmat M.
Samik-Ibrahim, Marshall Rose, Hector Santos, Jochen Topf, Greg
Vaudreuil, Patrick Cain, Paul Hoffman, Vijay Gurbani, and Hans
Lachman.
Diligent early proof-reading was performed by Bruce Lilly. Diligent
professional technical editing was provided by Susan Hunziker.
The final stages of development for this document were guided by a
design team comprising Alexey Melnikov, Pete Resnick, Carl S.
Gutekunst, Jeff Macdonald, Randall Gellens, Tony Hansen, and Tony
Finch. Pete Resnick developed the final version of the section on
internationalization.
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